1. Field of the Invention
The present invention relates to solid waste pulpers and, more particularly, to a solid waste pulper with an improved structure for controlling the solid waste processing rate.
2. Description of the Related Art
Solid waste pulpers are used to process cellulose-based and food waste products into small particles that, when combined with water, form a pumpable slurry. Pulping or processing is accomplished by feeding the waste products and water into a chamber containing electric motor driven rotating blades and fixed blades and passing the processed material through a particle sizing device. The ratio of solid waste to water must be kept below 2% solids by weight to ensure pumpability, and solid particle sizes must be kept small (small enough to pass through 1/4" holes).
Prior methods of controlling the solid waste processing rate relied upon controlling the water level in the tank within a very narrow range by one of two methods, as described below.
The first method, which can only be used for small units, employs a weir or standpipe to maintain water level and minimizes the feed rate with a restricted feed port. This adds size and weight to the machine and can cause discharge line clogs if trash is fed too fast or if pulp collects in the weir.
The second method, which is used for large pulpers or for units that require pumping, uses a centrifugal type pump to discharge the slurry and maintains water level by using a level sensing device to periodically add surplus water to the tank when the water level reaches a low set-point. This second method does not use a restricted feed port, but relies on measurement of the draw of electric motor current to provide feedback to the operator if trash is fed too fast. Level sensing devices are not reliable for this process because of several factors including vortex dynamics, fouling of sensor ports with pulp, and vibration. The feed rate control method is not sufficiently responsive to prevent slurry concentrations from exceeding pulpable levels (2% solids by weight, maximum) since sudden increases in feed rate result in water feed being reduced and the pump will, if starved, pull unpulped material through the particle sizing devices.
The above described prior art methods are particularly disadvantageous because the processing rate of the pulper is limited by restrictions on the solid waste feed and the discharge flow rate.